1. Field
The present invention relates generally to electronics, and more specifically, to telecommunications using a tunable oscillator.
2. Background
Consumer demand for mobile wireless services has led to the development of an ever increasing number of cellular networks. One such network is based on code division multiple access (CDMA) technology which supports wireless voice and data services using spread-spectrum communications. In spread-spectrum communications, a large number of signals share the same frequency spectrum and, as a result, provide a high level of user capacity. This is achieved by transmitting each signal with a different pseudo-noise (PN) code that modulates a carrier, and thereby, spreads the signal. The transmitted signals are separated in the receiver by a correlator that uses a corresponding PN code to despread the signal. The undesired signals, whose codes do not match, are not despread and contribute only to noise.
A competing network which has become the defacto standard in Europe and Asia is Global System for Mobile Communications (GSM) technology. Unlike CDMA, GSM uses narrowband time division multiple access (TDMA) technology to support wireless voice and data services. Other popular networks that have evolved over the years using TDMA technology include General Packet Radio Service (GPRS) and EDGE, both which support high speed data services. These networks may be dispersed throughout the geographic landscape, each with its own unique set of protocols, services, and data rates.
Today, wireless communication devices are being deployed with technology that supports multiple cellular networks. Typically, these devices are equipped with a dedicated receiver for each network. A local oscillator (LO) circuit may be used to provide a stable reference signal to each receiver. The stable reference signal may be used by each individual receiver to recover information signals from a high frequency carrier. The LO circuit is typically implemented with a crystal oscillator driving several frequency multiplier circuits. The frequency multiplier circuits may be individually programmed to provide a reference signal to each receiver at the proper frequency. In order to maintain good receiver performance, a highly accurate and stable crystal oscillator is often employed. Alternatively, a tunable oscillator, such as a voltage controlled temperature compensated crystal oscillator (VCTCXO) may be used. A frequency tracking loop may be used to tune the oscillator to compensate for manufacturing tolerances, Doppler frequency shifts, and drift.
In more advanced systems, wireless communication devices may be equipped with a Global Positioning System (GPS) receiver. GPS is part of a satellite based navigation system developed by the United States Department of Defense. It provides global coverage with navigational capability under various environmental conditions. In a fully operational GPS, the entire surface of the earth is covered by up to twenty-four satellites dispersed in six orbits with four satellites in each orbit. A GPS receiver in the wireless communications device uses signals modulated by a pseudo-random-noise (PRN) code from multiple satellites to pinpoint its exact location on earth. The raw data generated from the GPS receiver may be used for various applications. By way of example, the raw data may be plugged into map files stored in memory.
To improve the economic viability of these wireless communication devices, the GPS receiver often shares a common LO circuit with the cellular receivers. The problem with this approach is that the performance of the GPS receiver may be degraded if the crystal oscillator in the LO circuit is being tuned by the frequency tracking loop during GPS operation. Accordingly, there is a need for an innovative approach that can be used to tune a crystal oscillator in an LO circuit without degrading the performance of the GPS receiver.